By Jay Shah, Technical Team, Beta Diamond Products
For most mounted metallographic samples, grind 240, 400, then 600 grit ANSI silicon carbide paper, one minute per paper under running water, rotating the sample 90 degrees between grits. That lands you near 15 micron scratch depth, ready for 9 micron diamond. The complication, and the most common ordering mistake we see customers make, is that the same grit number means different particle sizes depending on which standard printed it. American papers use ANSI (also called CAMI) numbers. European papers use FEPA numbers prefixed with P. (Japan uses a third system, JIS; at these sizes it sits close to FEPA.) Through about 180 grit [P180] the two Western systems track closely. Above 180 they diverge, and the finer you go, the worse it gets.
The conversion chart
Read across a row for the equivalent grade in the other system. The micron columns are the real anchor, because micron numbers do not lie. The values below are the standard midpoints, ANSI/CAMI to ANSI B74.18-1996 and FEPA P to FEPA 43-GB-1984, the same sources the major brands chart from. A few FEPA grades (P320, P600, P4000) have no exact ANSI equivalent.
| FEPA P grade (Europe) | FEPA micron | ANSI / CAMI grade (USA) | ANSI micron |
|---|---|---|---|
| P60 | 269 | 60 | 268 |
| P80 | 201 | 80 | 188 |
| P100 | 162 | 100 | 148 |
| P120 | 127 | 120 | 116 |
| P180 | 78 | 180 | 78 |
| P240 | 58.5 | 220 | 66 |
| P280 | 52.2 | 240 | 51.8 |
| P320 | 46.2 | — | — |
| P360 | 40.5 | 280 | 42.3 |
| P400 | 35 | 320 | 34.3 |
| P500 | 30.2 | 360 | 27.3 |
| P600 | 25.8 | — | — |
| P800 | 21.8 | 400 | 22.1 |
| P1000 | 18.3 | 500 | 18.2 |
| P1200 | 15.3 | 600 | 14.5 |
| P1500 | 12.6 | 800 | 12.2 |
| P2000 | 10.3 | 1000 | 9.2 |
| P2500 | 8.4 | 1200 | 6.5 |
| P4000 | 5.0 | — | — |
Values are nominal midpoints; manufacturer-specific sizes vary by 10 to 20 percent. Final polishing abrasives, diamond, colloidal silica, alumina, are graded in microns only and have no grit number.
Read the 400 row again
ANSI 400 is roughly P800. A lab that restocks from a European supplier and grabs P400 thinking it matches their old 400 grit paper just doubled their particle size mid sequence, and every sample that week carries mystery scratches nobody can explain. This mistake is common; we field the conversion question constantly, and it is the single biggest reason we anchor everything to microns. When in doubt, ignore the grit number entirely and compare micron sizes.
Why this sequence works
The working rule for any abrasive progression: each step no more than about half the particle size of the one before. 240, 400, 600 ANSI walks roughly 52 to 22 to 15 microns, comfortably inside that ratio, and 600 grit at about 15 microns hands off cleanly to 9 micron diamond. Add 320 grit between 240 and 400 if your material scratches stubbornly. Skip steps and the finer paper spends its life chasing scratches it is too small to remove efficiently.
The 90 degree rotation between grits is not ritual. Turning the sample makes the previous paper's scratches run perpendicular to the new ones, so you can see, not guess, when the old scratches are fully gone. Grind until they are, then thirty seconds more for the deformed layer below.
Technique notes that matter more than the grit number
Water always: it flushes swarf, keeps the paper cutting, and prevents frictional heating that alters microstructure. Moderate pressure: pressing harder embeds particles in soft metals and deepens deformation everywhere. Fresh paper cuts, worn paper rubs: SiC papers dull within minutes, and a dulled paper burnishes instead of cutting. Papers are the cheapest consumable in the lab; change them early. And wash everything when grinding ends, sample, holder, hands, before anything touches a polishing cloth.
We stock SiC papers and discs across the full range, plain and adhesive backed, with micron sizes stated on the package so the chart above becomes someone else's problem. Free samples available.